*[PATCH 4.4 64/88] fork: record start_time late
2019-01-11 14:07 [PATCH 4.4 00/88] 4.4.170-stable review Greg Kroah-Hartman
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2019-01-11 14:08 ` [PATCH 4.4 63/88] scsi: zfcp: fix posting too many status read buffers leading to adapter shutdown Greg Kroah-Hartman
@ 2019-01-11 14:08 ` Greg Kroah-Hartman
2019-01-11 14:08 ` [PATCH 4.4 65/88] hwpoison, memory_hotplug: allow hwpoisoned pages to be offlined Greg Kroah-Hartman
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From: Greg Kroah-Hartman @ 2019-01-11 14:08 UTC (permalink / raw)
To: linux-kernel
Cc: Greg Kroah-Hartman, stable, Jann Horn, Tom Gundersen,
David Herrmann, Linus Torvalds
4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: David Herrmann <dh.herrmann@gmail.com>
commit 7b55851367136b1efd84d98fea81ba57a98304cf upstream.
This changes the fork(2) syscall to record the process start_time after
initializing the basic task structure but still before making the new
process visible to user-space.
Technically, we could record the start_time anytime during fork(2). But
this might lead to scenarios where a start_time is recorded long before
a process becomes visible to user-space. For instance, with
userfaultfd(2) and TLS, user-space can delay the execution of fork(2)
for an indefinite amount of time (and will, if this causes network
access, or similar).
By recording the start_time late, it much closer reflects the point in
time where the process becomes live and can be observed by other
processes.
Lastly, this makes it much harder for user-space to predict and control
the start_time they get assigned. Previously, user-space could fork a
process and stall it in copy_thread_tls() before its pid is allocated,
but after its start_time is recorded. This can be misused to later-on
cycle through PIDs and resume the stalled fork(2) yielding a process
that has the same pid and start_time as a process that existed before.
This can be used to circumvent security systems that identify processes
by their pid+start_time combination.
Even though user-space was always aware that start_time recording is
flaky (but several projects are known to still rely on start_time-based
identification), changing the start_time to be recorded late will help
mitigate existing attacks and make it much harder for user-space to
control the start_time a process gets assigned.
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/fork.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1411,8 +1411,6 @@ static struct task_struct *copy_process(
posix_cpu_timers_init(p);
- p->start_time = ktime_get_ns();
- p->real_start_time = ktime_get_boot_ns();
p->io_context = NULL;
p->audit_context = NULL;
cgroup_fork(p);
@@ -1573,6 +1571,17 @@ static struct task_struct *copy_process(
goto bad_fork_free_pid;
/*
+ * From this point on we must avoid any synchronous user-space
+ * communication until we take the tasklist-lock. In particular, we do
+ * not want user-space to be able to predict the process start-time by
+ * stalling fork(2) after we recorded the start_time but before it is
+ * visible to the system.
+ */
+
+ p->start_time = ktime_get_ns();
+ p->real_start_time = ktime_get_boot_ns();
+
+ /*
* Make it visible to the rest of the system, but dont wake it up yet.
* Need tasklist lock for parent etc handling!
*/
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